Executive Summary

Our reverse engineering team project product was a Weed Eater gas-powered line trimmer. We were asked to document our disassembly, analysis, and reassembly of the product.

Introduction

Disassembly Product

Our product was a 1989 Weed Eater gas-powered line trimmer, model LT7000. It is powered by a single cylinder two stroke engine, and is used to trim off unwanted weeds and plants.
We were unable to test it prior to disassembly because of regulations and safety concerns with fuel.

Before Disassembly

Purpose

The purpose of a gas-line trimmer is to cut, trim, and shear different types of grass, weeds, and small plants using a flexible line. This line is rotated about a reel/head powered by an internal motor. Connected to a shaft, the motor creates enough torque to make the line functional. The line then rotates and the purpose of the gas-line trimmer is performed. Both potential and kinetic energies are used here. More specifically, elastic potential energy and rotational kinetic energy drive the trimmer. A coil spring contains elastic potential energy, which is then transformed into rotational kinetic energy in the reel.

Operation

Because the trimmer requires gas, we were unable to operate and test the product. However, from prior knowledge, a typical gas-line trimmer has a small engine that spins a shaft. Ours appears to spin a flexible one, which connects to the head. The head contains three slots for cutting "fingers" that flare out when subjected to rotation. These fingers are responsible for trimming lawns, weeds, etc.

Components/Material

Without disassembling the product, the gas-line trimmer appears to be made up of 30-35 components. Because it is gas-powered, it will contain more parts than an electric trimmer. These components are all made up of a variety of different materials. The most visible and most common material is molded plastic. This material can be found in components such as the housing, head guard, trigger, handle, gas tank and smaller plastic components located inside the housing. The trimmer also contains a shaft made of aluminum. The line used is made of nylon but contains three individual lines, each thick in diameter to provide more strength and durability. All screws, nuts, and fasteners, along with a coil spring, are made of metal, most likely steel. Assuming the trimmer contains a piston-cylinder device to drive the motor along with the motor itself, we can assume the two to be made of steel as well. The startup rope is most likely made from nylon or polyester. Knowing several of the components and the materials they are made from, we can now begin the disassembly portion and compare what we saw with what we predicted.

Assembly

Assembly was fairly straightforward, though some of the steps had varying levels of difficulty. The assembly steps were as follows:

Re-house the coil spring into the front plastic housing

Very difficult to correctly position spring. The housing process took more than a dozen trials.

Tools used:

Needle-nose pliers

Pliers

Flathead screwdriver

Small butterfly binder clips

Time spent: approximately 12+ man hours.

Fit the reel with nylon string into place.

Difficult to correctly align with internal coil spring.

Tools used:

Phillips-head screwdriver

Needle-nose pliers

Time spent: roughly 15 minutes on successful trial.

Piece engine together, attach to reel

Fairly easy to fit together from spark plug to piston, some lubrication lacking.

Grease applied where needed, engine needs two-stroke fuel priming.

Tools required:

Hex key

Adjustable wrench

Time spent: 20 minutes after some trial and error with confusing bolts.

Piece together choke

Moderately difficult due to small parts and spring-loaded assembly, but quick to fix small errors.

Tools required:

Hex key

Needle-nose pliers

Phillips-head screwdriver

Time spent: 30 minutes

Assemble housing

Very easy once we identified bolt/screw positions.

Tools required:

Hex key

Phillips-head screwdriver

Time spent: 15 minutes

Attach guard, reel and handle to pole

Easy to piece together and adjust.

Tools required:

Wrench

Pliers

Phillips-head screwdriver

Time spent: 10 minutes

Attach pole to housing, and throttle cable to choke.

Easy to piece together.

Tool required:

Hex key

Time spent: 5 minutes

After Assembly

Final Function

We were once again unable to secure a source of fuel for testing our Weed Eater.

Concluding Remarks

After more than 30 man hours of work, we were successful in reassembling our Weed Eater. In retrospect, this project was straightforward in theory, though difficult in practice. Disassembly was not very complicated, and we were able to finish within two hour-long sessions. There were a large number of parts, some of which we grouped together for ease of differentiation among their types. The most hazardous part of disassembly was removing the coil spring, which released tension on us very suddenly. Aside from that, it was a smooth process.

Reassembly, however, was a different story. It was easy to understand what we needed to do by simply reversing the process according to the extensive pictures we took. The initial steps proved to be a challenge for us. We spent far too much time trying to re-coil the coil spring: three meetings with four people per meeting, dozens of successful re-coils but unsuccessful placements, many minor fixed damages, a call to Weed Eater support, and much frustration later, we were able to re-seat the coil spring where it was supposed to go.

Beyond this major reassembly setback, the rest of our operation went smoothly. We decided to add a touch of grease to let the flywheel spin more freely. Cosmetically, the Weed Eater looks markedly similar to its initial state, with the same two missing fingers as it had when we began. Functionally, the Weed Eater seems to behave the same way as far as we know. Perhaps the only noticeable difference is the pull cord's incomplete retraction, our final penalty for our extended troubles with the coil spring. Assuming one can pull fast enough, the cord will still work, at least in theory. The single cylinder will need fuel/oil priming before its first reassembled run-- we were not able to add any sort of oil or gas to the product as stated previously. The pull string retracts and spins the flywheel, and upon a pull, one can feel and hear the piston compressing the cylinder's air.

In retrospect, this project was much more difficult and time consuming than we initially imagined. For future disassembly projects, we would not recommend this product without a simpler way to safely re-house and re-coil the coil spring.

Engineering Analysis

“Explain how analyses could be used to design and test your product (or some of its components). What type of basic engineering models could be used? Could you use estimates or would you need very precise models?”

For this issue, you need to discuss what kind of engineering models you could use to design your product. We went through a number of different models types in class (semantic, graphical, analytical, physical). While you could comment on the use of semantic, graphical, and physical models in the design of your product, I am more interested in the use of analytical (mathematical) models. You should address what kind of mathematical engineering models could be used to design your product. You need to consider what kind of engineering science principles are critical to the operation of your product. Some examples could include fatigue models, circuit models, power transmission models, thermodynamics/heat transfer models, fluid flow models, static loading models, material stress/strain models, etc. You don’t need to develop those models, but should discuss what kind of models are necessary and explain why you think those models are necessary and how they could be applied to your product.

References

APA Style
You must use this format (It's easier than MLA, so don't worry).